Dusty Disks and the Infrared Emission from AGN

20 Pages·2003·0.25 MB·English

Dusty Disks and the Infrared Emission from AGN
Published in Theory of Accretion Disks, 1989
DUSTY DISKS AND THE INFRARED
EMISSION FROM AGN
E. Sterl Phinney
Theoretical Astrophysics, 130-33 Caltech
Pasadena, CA 91125 U.S.A.
ABSTRACT. The distortions inferred in the gaseous disks of active galaxies suggest that a significant,
and possibly dominant fraction of the 1-1000 µm radiation observed from AGN must be thermal
emission from gas and dust heated by the central source. We report calculations of the growth and
sublimation of dust grains in the outer parts of accretion disks appropriate to AGN. The thermal state of
the gas undergoes a sudden change at the radius where the dust sublimates. The outer portion of the
accretion disk radiates at 0.5-5 µm; free-free emission from gas whose dust has sublimated contributes to
the flux at 0.5-2 µm. If this thermal emission dominates the flux from radio-quiet quasars, it naturally
explains the frequency and depth of the universal minimum in F at 1014.5 Hz. Free-free emission
from the photoionized surface layers of the disk at larger radii produces a radio flux at 1011 Hz
comparable to that observed in radio-quiet quasars. The far-infrared and submillimeter emission from
radio-quiet quasars and Seyfert galaxies is more naturally interpreted as reradiation by dust, than as
nonthermal emission from the inner accretion disk.
Table of Contents
INTRODUCTION
DUST AND WARPS
OUTSKIRTS OF THE GALAXY AND SUBMILLIMETER SPECTRUM
TRANSITION DISK
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Dusty Disks and the Infrared Emission from AGN
SURFACE PHYSICS AND RADIO EMISSION
SUBLIMATION AND THE TEMPERATURE GAP
CONCLUSIONS
REFERENCES

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Dusty Disks and the Infrared Emission from AGN

1. INTRODUCTION
It is generally accepted that emission from heated dust produces the steep far-infrared continua of Seyfert
2's and the IRAS warm galaxies. It is also generally acknowledged that the rapidly variable infrared
emission in the BL Lacs and optically violently variable quasars must be produced by non-thermal
processes. Both thermal and non-thermal emission evidently occurs in nature, in objects of comparable
luminosities. The controversy over the nature of the emission in radio-quiet quasars is thus not over
whether either emission process is physically possible or plausible. Both are, and both are probably
present at a significant level in all objects. The question is simply which happens to predominate in radio-
quiet quasars.
In most optically selected quasars, the 1-100µ infrared luminosity comprises 10-50% of the bolometric
luminosity (Sanders et al. 1989). Since the energy liberated per octave in radius in an accretion disk
scales as r-1, the high relative infrared luminosity requires that the ultimate source of energy for the
infrared radiation be within a few times the inner radius of the accretion disk. If the infrared radiation is
emitted from a region comparable in size to that of its energy source, it must be nonthermal: blackbody
limits on the source size at 1 µ and at 60 µ are respectively > 0.1 pc and > 100 pc for the most luminous
sources in figure 1a and > 0.003 pc and > 10 pc for the least luminous. If the infrared radiation is thermal,
and therefore emitted at large radii, it must be reprocessed energy transported to the emission radii by
radiation or by mechanical means (e.g., a jet).
Dust heated locally by stars may contribute to some of the infrared emission from quasars. It is unlikely,
however, that this dominates in the majority of sources. For stars to produce the typical infrared
luminosity of 1013 L = 0.5 M c2 yr-1 would require a star formation rate of at least 500 M yr-1 (this
lower limit assumes that only massive O stars are formed; a normal IMF would require a rate
approximately 10 times higher). Over the ~ 108 yr lifetime of a quasar, > 5 x 1010 M of gas would have
to be processed in massive stars. Dust would form from the metals produced. So much gas and dust,
pushed to high latitudes by supernova explosions, would inevitably absorb and reradiate much of the
luminosity from the central source. But the optical and ultraviolet radiation from quasars, variable on
timescales 10 yr (Usher 1978) must come from such a central relativistic source. Since its luminosity
is comparable to the infrared luminosity, we conclude that reradiation from gas and dust heated by it
would necessarily be at least comparable to anything contributed by stars. In what follows we therefore
ignore the heat input from stars, except insofar as they provide a natural minimum dust temperature 25
K.
Quasars and Seyfert galaxies appear to be located in galaxies amply supplied with interstellar medium.
The disks of gas and dust in normal galaxies exhibit warps on all scales, and would intercept and re-
radiate ~ 10% of the luminosity from a central source. Several lines of evidence suggest that the warps
are even more severe in Seyferts and quasars, so that an even larger fraction of the central luminosity
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would be reradiated. Provided most of the reradiating material is located in an optically thick disk, the
central source will never appear absorbed or reddened - either we have a clear line of sight, or the source
is entirely occulted. This picture is thus consistent with the absence of reddening or absorption by broad
emission line clouds in quasars.
We describe below the opacities and physical state of gas in the disks at distances from 10-3 pc to 104 pc
from the central source. Dust grains can form and grow in the disk within 1 pc. At much smaller radii,
however, even graphite grains will sublimate. When this occurs, the gas loses its primary coolant, and
heats until it reaches a new thermal equilibrium at 104 K. The superposition of emission from radii
inside and outside this transition point naturally explains the minimum in L at = 1014.5 Hz
observed in most quasars. The characteristic scale length of dust in galaxies (~ 2-10 kpc) naturally
explains both the frequency and steepness of the drop in L at = 1014.5 at 1012 Hz. The
normalization of infrared luminosity relative to the UV and X-ray luminosities of quasars is consistent
with expected covering factors and space-densities. Free-free emission from the photoionized zones on
the illuminated surfaces of the disk naturally provides a flat-spectrum radio flux comparable to that
observed in many quasars (Antonucci & Barvainis 1988), and may contribute to the optical continuum
emission. It appears therefore that the emission at wavelengths 1-1000 µm from Seyfert galaxies and
quasars other than OVV's is naturally explained as thermal reradiation from the nuclear disk and the
interstellar medium of the host galaxy. Although non-thermal emission from the central source may
contribute in some objects at some times, a significant contribution from thermal emission seems
unavoidable.

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2. DUST AND WARPS
Most previous discussions of dust and gas in the host galaxy of an active nucleus have concentrated on
spherically symmetrical distributions of dust (Rees et al. 1969, Bollea & Cavaliere 1976, Barvainis 1987 -
the latter also considered conical sectors, or have concentrated on their effects on emission line clouds:
Davidson & Netzer 1979, MacAlpine 1985 and references therein). A few have considered disk-like
distributions (Begelman, McKee & Shields 1983, Begelman 1985, Shlosman & Begelman 1988), but
have assumed the disks were planar and relatively thin, so that they intercepted only a small fraction (
1%) of the luminosity of the central accretion disk. Neither of these distributions is very likely. The gas
and dust most probably lie in what could crudely be described as a heavily warped (and probably clumpy)
disk. Beyond about a kiloparsec radius, the orbital time t 108 (r / 3 kpc) yr is so long that gas captured
orb
or disturbed by a recent interaction with another galaxy will not have had time to settle into the preferred
plane of the host galaxy's potential in a quasar lifetime (~ 108 yr). Since quasars seem commonly to be
involved in interactions or mergers (Hutchings 1983, MacKenty & Stockton 1984), large warps and
streamers are to be expected. The warped disk of NGC 5128 (Centurus A) is a famous example. Moving
inwards, warps on kiloparsec scales can be produced by counter rotating bars (Vietri 1986), by continuing
infall of gas (Ostriker & Binney 1989), and by Kelvin-Helmholtz instability as the disk rotates through a
pressure-supported corona (Gunn 1979) Though such warps are difficult to detect in external galaxies
which are not nearly edge-on, let alone in quasars, the gas at ~ 3 kpc in our own Galaxy is tilted by about
15° with respect to the plane defined at larger radii (Vietri 1986), and most well-studied Seyfert galaxies
have strong kiloparsec-scale bars (Adams 1977).
The same warp-inducing processes can operate on scales of parsecs. The molecular torus extending from
2-8 pc from our Galactic center is tilted by ~ 15° (in the opposite direction from the 3 kpc warp!) with
respect to the plane defined by the stars. The parsec-scale dust torus in NGC 1068 (Antonucci & Miller
1985) has its axis at right angles to the kiloparsec-scale disk of gas and stars (Wilson & Ulvestad 1982),
and the axes of similar tori inferred to exist in other Seyferts make random angles to the minor axes of the
disks of their host galaxies (Unger et al. 1987, Haniff et al. 1988).
We conclude that warps are common and substantial enough to allow the nuclear continuum to illuminate
the dust and gas on scales from 1-104 parsecs. The broad-line region (BLR) will be obscured by this dust
over a range of viewing angles comparable to the covering factor of the dust disk. Except in the case that
the narrow line gas is cospatial with the dusty disk in a symmetrical warp, it is difficult to prevent the
narrow line region from being visible from some viewing angles when the BLR is obscured. This occurs
for a fraction of sky of order the covering factor of the dust disk extending beyond the NLR: typically ~
0.05-0.1. Since molecular clouds are larger than the 0.1-1 pc scale of the BLR, the same fraction of
quasars with obscured broad line regions would be expected even if the dust were in clouds at high
latitudes rather than in a disk. These objects would appear as ``quasar-2's'' (by analogy to Seyfert 2's).
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Such objects have been rare in optical surveys, but appear common in infrared-selected samples (Sanders
et al. 1988b). In radio samples they may masquerade as narrow line radio galaxies (Scheuer 1987, Barthel
1989).
We now examine the state of dust in a warped disk illuminated by the central accretion disk of a quasar,
and its possible relevance to the infrared and submillimeter spectra of quasars. We postpone to section 5 a
discussion of the vertical structure of an illuminated disk, and the expected optical and radio emission
therefrom.
The equilibrium temperature T of dust grains of characteristic radius a 20 Å at distance r from a
g
radiation source of luminosity density L (erg s-1 Hz-1) is determined implicitly by
(1)
where Q ( ) is the absorption efficiency (cross section in units of a2), at frequency . Graphite grains
abs
with a ~ 0.1 µm are transparent to X-rays with h > 0.4 keV and for 0.1 keV < h < 0.28 keV (K-edge of
Carbon), so Q ( ) a at those energies. At wavelengths < 2 a where the grain is not transparent
abs
Q 1. At longer wavelengths > 2 a the grain becomes a weakly coupled antenna, Q (2 a /
abs abs
) f( ) where f( ) depends on the dielectric tensor and shape of the grains (fits for various grain
compositions and shapes, and discussion of the Mie scattering range can be found in Martin 1978, Draine
& Lee 1984, Wright 1987, and references therein). Observations of galactic dust indicate that outside
resonances f( ) 1- , with 1 2 (Whittet 1988).
In quasars, most of the energy from the central source is emitted at frequencies for which Q ( ) 1,
abs
and reradiated at wavelengths > 2 a. Hence if we ignore heating and cooling of grains by collisions
with atoms (generally weak), the temperature of directly illuminated grains is given approximately by
(2)
where < Q (T) > is the Planck-averaged absorption efficiency (cf. Draine 1981), < Q (T) > T .
abs abs g
Crudely, therefore, T [L / (r2 a)]1/(4+ ). At a given distance from the source, the smallest grains in
g
thermal equilibrium (~ 30 Å) will be about a factor of 2 hotter than the largest grains commonly
considered (~ 0.3 µm).
The heat capacity of grains with a 20 Å is so low that their temperature fluctuates, being significantly
affected by the absorption of a single UV photon. Grains deeper in the dusty disk will not be exposed
directly to UV radiation from the central source, but to longer-wavelength re-emission from shielding gas
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and dust. This shielded dust will have a temperature nearly independent of a, and slightly lower (by a
factor ~ (T / 2500 K)1/(4+ )) than that of the directly illuminated grains of temperature T .
re re
The emission from dust at temperature T at long wavelengths scales as 2+ T , peaks very
g g
sharply at h ~ (3 + )kT ( ~ 30 T -1 µm, where T = 100 T K) and declines exponentially at higher
g 2 g 2
frequencies. Except for the small grains of fluctuating temperature, which can contribute high frequency
emission from regions where the equilibrium temperature is low, most emission at frequency v will come
from the radius where the equilibrium dust temperature T ~ h / (3 + ) k, i.e., from a radius r L1/2 -
g
(4+ )/2, and the flux of radiation at that frequency will (for an isotropic central source) be proportional to
the fraction of the sky at the central source covered by dust at the appropriate radii. This radius-frequency
scaling can cause curious effects. At frequencies where is large (e.g., 1 < < 7 µm, where 1.7 -
Whittet 1988), the temperature is nearly independent of radius, enhancing the probability of having a
large warp at an appropriate radius, and hence a large sky covering factor and a large ~ 3 µm flux (see
figure 2). This may be the cause of the ``3-5 µm bumps'' commonly observed in AGN (Edelson &
Malkan 1986).

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3. OUTSKIRTS OF THE GALAXY AND SUBMILLIMETER SPECTRUM
At low frequencies is also large ( -> 2 when 0-frequency isotropic conductivity dominates the
dielectric tensor), so that the temperature changes only slowly with radius at the outskirts of the galaxy (~
3 - 30 kpc) where the molecular gas distribution of galaxies rapidly becomes very patchy. The
characteristic temperature at the outskirts of a typical galaxy would be ~ 30 - 50 K (see figure 1).

Figure 1. Temperature of directly illuminated 0.1 µm
graphite grains as a function of distance from a central UV
source of luminosity L = 1046 erg s-1. Planck-averaged
UV
absorption efficiencies used are from Draine & Lee 1984. For
sources of other luminosities and grains of other sizes, the
temperature in the flatter portions of the curve (T > 300 K, T
< 60 K) scales roughly as (L / a)1/6, and in the steeper
UV
portion (60 K < T < 300 K) as (L / a)1/4. For a galaxy disk
UV
with a smooth logarithmic warp, dust at the given
temperatures contributes predominately to the flux at
wavelengths marked on the right.
A dust layer wil absorb UV flux incident at an angle to the normal of the layer if its column density >
10-2 cos g cm-2. With a Galactic gas-to-dust ratio spread over a disk of radius r this column
kpc
corresponds to a mass of gas M 2 x 108 cos r2 M . The gas masses in nearby AGN are inferred
H kpc
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from observations of CO (which of course depletion relates more directly to the dust than to M !) to be
H
of order 108-1010 M (Sanders, Scoville, & Soifer 1988a), and even in a young galaxy such as might
surround a high redshift quasar it is unlikely that there would be more than ~ 1011 M of processed gas.
Consequently the disk will become optically thin beyond a few kpc (it could become thin at a smaller
radius if most of the dust is clumped into clouds with >> 10-2 cos g cm-2, and such clouds at larger
radii could preserve dust in neutral cores, but their covering factor would necessarily be very small).
Since the covering factor of dusty material at 20 K (~ 100 kpc) is expected to be very small, the spectrum
of dust reradiation will be characterized by a rather well-defined minimum temperature, and should roll
over sharply at wavelengths 200 µm with F 2+ at longer wavelengths (as for similar reasons
it is observed to do in starburst galaxies and Galactic H II regions - cf. Telesco & Harper 1980). The
precise form of the spectrum at 60 µm < < 200 µm will vary depending on the covering factor at large
radius, which could be enhanced by the presence of companion galaxies (whose starlight maintains a
minimum dust temperature ~ 20 K!), tidal tails, and the like.
As we discuss in section 5, at frequencies < 1011 Hz, free-free emission from photoionized gas at the
illuminated face of the disk will dominate the spectrum. Figure 2 shows the spectrum of continuum
reradiation from gas and dust in an exponential disk with a logarithmic warp (d(covering factor) / d ln r
= const). To illustrate how material at large radii can affect the far infrared and submillimeter spectrum,
we show the effect of adding reradiation from a 2 x 20 kpc slab of dust extending from 10 to 30 kpc
(which could represent a companion galaxy or a tidal tail).

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Dusty Disks and the Infrared Emission from AGN
Figure 2. Spectrum of reradiation from dust and
photoionized gas in a warped disk surrounding a
quasar with L = 1046 erg s-1. The solid line
UV
represents a disk containing 0.1 µm graphite dust
having C = d(covering factor) / dln r = 0.1 exp(-r /
10kpc). Note the ``5 µm bump.'' The dot-dashed
line shows the result of adding 0.03 to C for 10 < r
< 30 kpc - representing a tidal tail or companion
galaxy. The dashed line shows the spectrum of
reradiation from (very large) grains assumed to
radiate as black bodies, with the same covering
factor distribution as for the solid line. The dotted
line shows the contribution of free-free emission
from the photoionized zones above the dust. There
is no freedom in its normalization relative to the
dust spectra.

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